1. Field of Invention
The field of the currently claimed embodiments of this invention relates to optical coherence tomography systems, and more particularly to axial motion-compensated optical coherence tomography systems.
2. Discussion of Related Art
Optical coherence tomography (OCT) has been viewed as an “optical analogy” of ultrasound sonogram (US) imaging since its invention in early 1990's (D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence tomography,” Science, vol. 254, pp. 1178-1181, 1991). Compared to the conventional image-guided interventions (IGI) using modalities such as magnetic resonance imaging (MRI), X-ray computed tomography (CT) and ultrasound (US) (T. Peters and K. Cleary, Image-Guided Interventions: Technology and Applications, Springer, 2008), OCT has much higher spatial resolution and therefore possesses great potential for applications in a wide range of microsurgeries, such as vitreo-retinal surgery, neurological surgery and otolaryngologic surgery.
As early as the late 1990's, interventional OCT for surgical guidance using time domain OCT (TD-OCT) at a slow imaging speed of hundreds of A-scans/s has been demonstrated (S. A. Boppart, B. E. Bouma, C. Pitris, G. J. Tearney, J. F. Southern, M. E. Brezinski, J. G. Fujimoto, “Intraoperative assessment of microsurgery with three-dimensional optical coherence tomography,” Radiology, vol. 208, pp. 81-86, 1998). Thanks to the technological breakthroughs in Fourier domain OCT (FD-OCT) during the last decade, ultrahigh-speed OCT is now available at >100,000 A-scan/s. For example, see the following:                B. Potsaid, I. Gorczynska, V. J. Srinivasan, Y. Chen, J. Jiang, A. Cable, and J. G. Fujimoto, “Ultrahigh speed Spectral/Fourier domain OCT ophthalmic imaging at 70,000 to 312,500 axial scans per second,” Opt. Express, vol. 16, pp. 15149-15169, 2008.        R. Huber, D. C. Adler, and J. G. Fujimoto, “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s,” Opt. Lett., vol. 31, pp. 2975-2977, 2006.        W-Y. Oh, B. J. Vakoc, M. Shishkov, G. J. Tearney, and B. E. Bouma, “>400 kHz repetition rate wavelength-swept laser and application to high-speed optical frequency domain imaging,” Opt. Lett., vol. 35, pp. 2919-2921, 2010.        B. Potsaid, B. Baumann, D. Huang, S. Barry, A. E. Cable, J. S. Schuman, J. S. Duker, and J. G. Fujimoto, “Ultrahigh speed 1050nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second,” Opt. Express, vol. 18, pp. 20029-20048, 2010.        W. Wieser, B. R. Biedermann, T. Klein, C. M. Eigenwillig, and R. Huber, “Multi-Megahertz OCT: High quality 3D imaging at 20 million A-scans and 4.5 GVoxels per second,” Opt. Express, vol. 18, pp. 14685-14704, 2010.        T. Klein, W. Wieser, C. M. Eigenwillig, B. R. Biedermann, and R. Huber, “Megahertz OCT for ultrawide-field retinal imaging with a 1050 nm Fourier domain mode-locked laser,” Opt. Express, vol. 19, pp. 3044-3062, 2011.        
For a spectrometer-based SD-OCT, an ultrahigh speed CMOS or CCD line scan camera based system has achieved up to 312,500 line/s in 2008 (Potsaid et al.); while for a swept laser type OCT, >20,000,000 line/s rate was achieved by multi-channel FD-OCT using a Fourier Domain Mode Locking (FDML) laser in 2010 (Wieser et al.).
Fourier-domain optical coherence tomography (FD-OCT) is a high-speed high-resolution three-dimensional imaging modality widely used in biomedical imaging. For OCT to find applications in the interventional imaging area, real-time image processing and display are required.
Optical coherence tomography (OCT) is a non-invasive, high speed, high-resolution, three-dimensional imaging modality that is widely being used for biomedical application [1, 2]. The real-time non-invasive depth-resolved imaging of tissue structure and flow information provided by OCT can be highly valuable information that can assist physicians in making real-time decisions during surgical procedures such as neurosurgery, tumor resection, microvascular anastomosis, and retinal microsurgery [2-10].
In many circumstances, it is more convenient to use a simple hand-held, manually-scanned probe to obtain OCT images of tissues and organs which might otherwise be inaccessible using standard mechanical scanning heads [6]. A hand-held image probe has the following advantages. First, it is small and lightweight, making it easy to access tight spaces. Second, surgeons are intimately familiar with hand-held instruments which can leverage the surgeons' experience and skills with little training. Third, a small hand-held instrument offers greater safety because the surgeon can more easily override or remove the instrument in cases of malfunction [11]. Finally it offers the surgeon great freedom to obtain any image size, for example a larger field-of-view compared to views constrained by apertures of scanning lenses or other endoscopic probes [12-15].
A hand-held probe, however, poses additional challenges over mechanically-rigid scanners. First, non-uniform motion of the probe during lateral manual scanning will cause image distortion and inaccuracy. Earlier work by Ahmed et al. and more recent work by our lab provide solutions to correct non-uniform scanning speed artifact using decorrelation of adjacent A-lines [6, 16]. Second, physiological tremor composed of low and high frequency amplitudes over 100 μm [17] would cause large motion artifacts in acquired images. Third, involuntary motions of a subject may also cause OCT imaging artifacts. Finally, the manual scan across and close to the target surface is highly risky—especially involving fragile tissue. For example, in the context of retinal surgery, the retina is only ˜350 μm for humans, and tearing them can permanently damage eyesight. Scanning while maintaining a larger distance between the probe tip and target surface is not an ideal solution since that degrades image quality and the imaging depth is typically limited to 3-5 millimeters. While motion is in the form of both axial and lateral directions, axial motion is the primary concern due to its direct effects on the image quality. There have been methods used to compensate for the sample surface topology and axial motion in OCT to keep a good system sensitivity and image range, for example the adaptive ranging technique for time-domain OCT (TDOCT) [18] and the reference mirror tracking method for spectral domain OCT (SDOCT) [19]. Common path optical coherence tomography (CP-OCT) is a simple, all-fiber-based technique that shares reference and probe arm which circumvents group velocity dispersion and polarization compensation [20]. It has been widely investigated for various medical applications [21-24]. CP-OCT has also recently been demonstrated for its effectiveness, simplicity, and integrability for surface topology and motion compensation [25] and smart surgical tools [26]. There thus remains a need for improved optical coherence tomography systems that provide motion compensation.